Multicathode phototube



May 27, 1947. R. w. ENGSTROM 2,421,192

MULTI-CATHODE PHOTOTUBE Filed May 29, 1943 INVENTOR HLF'H W. ENESTREIM ATTORNEY atente d May 27, 1947 MULTICATHODE PHOTOTUBE.

Ralph W. Engstrorn, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application May 29, 1943, Serial No. 489,000

6 Claims.

My invention relates to phototubes and particularly to phototubes incorporating a plurality of photo-electrically sensitive cathodes.

It has been-customary in conventional phototubes to provide two or more photocathodes sideby-side and facing in one direction for use either with a single or multiple light source. In certain applications, however, it is desirable to provide a structure suitable for use with a single light source proj ecting light on the light sensitive structure either from a wide angle or as a spot and to provide a circuit and phototube responsive to relative movement between the light spot and the tube for use as direction indicators. Such applications require very accurate spacing of the electrodes and a very rugged structure so that longitudinal movement and otation of the phototube with respect to the light spot or its origin produces no displacement of the tube parts either with respect to one another or with respect to the tube envelope.

An object of my invention is to provide a phototube and phototube structure wherein precise alignment of a plurality of photo-electrically sensitive cathodes may be obtained. It a is further object of my invention to provide a more sensitive photo-electric structure for the determination of relative movement of a light source with respect to said structure. It is a still further object to provide a phototube wherein a relatively small displacement of a spot of light incident on the sensitive structure provides a relatively great change in tube sensitivity. It is a still further object to provide a rugged photo-electric tube of the multi-cathode type wherein each of the oathodes is substantially equally sensitive, and a still further object is to provide a tube which is rugged, easy of manufacture and suitable for accurate determination of light source movement.

In accordance with my invention I provide a plurality of photo-electrically sensitive cathodes surrounding a centrally disposed tube axis and.

disposed such that the cathode areas projected on a plane normal to the axis are equal and are considerably smaller than the actual cathode areas, and such that the planes of the cathodes intersect at a point outside the tube envelope. In addition, each of the cathodes is insulated from the adjacent cathode so that each may be energized with respect to an anode surroundin and symmetrically disposed with respect to said cathodes and said axis. These and other objects, features and advantages of my invention will become apparent when taken in connection with the accompanying drawing wherein:

Figure 1 is a longitudinal view of a phototube made in accordance with my invention with a portion of the envelope cut away to show the internal structure, and

Figure 2 is a cross-sectional plan view of the tube of Figure 1 taken along the lines 2-2.

Referring to Figure 1, I have shown an envelope I having a stem or press 2 supporting my new and improved multi-cathode and electrode structurev axially of the envelope l by means of current carrying leads and supports sealed in the press 2. To support the plurality of cathodes I provide transverse insulating members substantially parallel one to another, between which the cathodes and the anode are supported. More particularly, I provide two insulating spacers such as an upper spacer 4 and a lower spacer 5 between which I.

support four electrically insulated cathodes 6. -It

will be appreciated that any plurality of-suchpreferably integral with the cathodes; extend,

the tabs being bent over on the under side of the spacer 4 as shown at 9. The lower spacer 5 is provided with similar apertures or slots H! to support the lower edges of the cathodes 6. Consequently, I provide on thecathodes 6, preferably integral with the lower edges thereof, a plurality of tabs H which extend through the corresponding apertures in the lower spacer 5. I have shown the cathodes as being of rectangular form with the integral tabs bent over and engaging the apertures in the upper and lower spacers, but it will be appreciated that these cathodes need not be of rectangular form but that they may be trapezoidal in shape with the base portion supported by the spacer 5. v I I As shown by the drawing, the points of support for the cathodes on the lower spacer 5 are more remote from the axis 3-3 than are the points of support on the upper spacer l, so that the surface of the cathodes effectively lies on or forms a truncated pyramidical surface whose apex is without the envelope l and lying on the axis 33. Thus in accordance with a limited teaching of my invention, I provide a multiple cathode structure surrounding a centrally disposed axis, the cathodes forming in efiect the side portions of a right truncated pyramid. However, I will use the term truncated pyramid to include a truncated cone wherein the side portions are curved and lie on or form a truncated conical surface whose apex is without the envelope and on the axis 33. Still further in accordance with my invention, I provide an anode common to all of the cathodes preferably lying equally distant from the longitudinal axis about which the cathodes are disposed. More particularly, as shown in the drawing, I provide an anode l2 preferably in the form of a ring surrounding the cathodes 6 at a greater distance from the axis 3-3 than the cathodes and in a position between the two transverse insulating support members such as the sp acers *4 and "5. The anode I2 is preferably supported by support wires I 3 extending substantially parallel "to the axis 3-3 and as far removed from the cathodes as possible. Thus as shown, especially in Figure "2, the support wires l3 may extend transversely from the press 2 and then parallel to the :axis 3-4; in a reg ion adiacen t at least two of the corners formed between adiaoent cathode planes. With such positioning, the support wires t3 produce the minimum of interference or unequal current collection with respect to the cathodes, while at' th'e same time being located without the path or light which, during operation, is incident on the '-cathodes such as along an optical path coincident with the aids 3 3.

With my improved construction or a multiple cathode phototube it will :be appreciated that each of the cathodes may be illuminated equally by a spot of lightconcontric with the said longitudinal axis, and that the cathodes for .such coincidence and with a circular spot receive an equall' a'm'ount or light assuming uniform distribution of light within the spot. However, with non-coincidence of the spot with the axis, the caithodes-receive unequal portions of the light and consequently "provide unequal photo-electric currents in the respective cathode circuits. The sensitivity of my device with respect to such movementof the incident light spot with respect to the cathodesis greatly increased by this dis- H position of the cathodes about the lateral surface of a truncated pyramid because the intercepted light variesat a relatively great rate for differences in-*the angle of incidence of light from a source near the apex of the pyrami'dical disposition of the cathodes.

"Phe cathodes may be of any of the well-known types such as the alkali metal treated silversilver oxide type or of the alkali metal treated antimony, arsenic 'or bismuth type depending upon the particular spectral range of the light source used in combination with the tube. Thus, for example, the cathodes 6 maybe of silver or silver plated base metal oxidized preferably by a glow I discharge in oxygen developed between the cathodes and the anode and treated with an alkali metal such as caesium. Following such oxidation, the cathode may'lbe treated .by libcrating caesium or other alkali metalsuch as from a pellet of alkali metal compound attached to an 'ac tiva'tor tab M :as well known in the art. Alternatively, the cathodes 8 'may be of a base metal :such as nickel, or of an oxidized chromium containing nickel :allow, coated with a film of antimony or .an equivaleat element such as :ar-

senic or bismuth. This latter type of cathode is particularly suitable for obtaining high photoelectric response when subjected to light over the blue and violet portions of the spectrum, whereas the former type of cathode is particularly suitable for light in the red and infra red portions of the spectrum. The antimony film on the cathodes, if the latter type is used, is similarly treated with an alkali metal such as caesium which may be liberated within the envelope from the activator tab I4.

I claim:

1. A phototube comprising an envelope, a plurality 0f cathodes and an anode within said envelope, said cathodes lying along surfaces forming a truncated pyramid whose apex is outside sai'd envelope.

2. A phototube comprising an envelope, a plurality of cathodes surrounding a centrally disposed axis, and an anode Within said envelope, catho'des llying along surfaces forming a truncated pyramid whose apex is outside said envelope and on-said centrally disposed axis.

3. -A phototube comprising an envelope enclosing :a plurality of planar cathodes surrounding a centrally disposed "axis, the extended surfaces of said cathodes intersecting said axis at a common point without the envelope and an anode surrounding said cathodes and :said axis.

4. .A phototube comprising an envelope, a pluralilty of photosensitive cathodes and an anode within said envelope, said cathodes lying along the s boun'ding lateral surfaces of a truncated pyramid whose'planes intersect to define a virtual apex oi? said pyramid without said envelope.

i5. A phototube comprising an envelope, a pair of electrode support members lying transversely of said envelope, a plurality of planar cathodes mounted about an axis within said envelope and between said electrode support members, said cathodes lying along the lateral surfaces of a truncated pyramid and an anode surrounding said cathodes between each thereof and said envelope.

6. A phototube comprising an'envelope, :a .pair of apertured electrode support emembers :lying transverselyiof saidenvelopa'azpluralityof planar cathodes mounted about 'an axis within said envelope, a plurality of integral tabs on each of said cathodes, said tabs engaging the apertures of said support members and being supported thereby, the apertures in :said support members being positioned with said cathodes inclined to said axis "by'substantially the same amount.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,999,646 'Witflmhns Apr. 30, 1935 72,070,178 1Pottenger,-Jr.,'et al. Feb. 9, 1935 2,207,356 Pierce July), 1940 FOREIGN PATENTS Number Country Date 340,527 Great .Britain .Dec. .29, 1930 .'3 7Q,-96-7 Great Britain .Apr. 11, 1932 

